It is well known in the prior art that natural fibers of cotton, wool and feathers can undergo reversible variation in form and percentage of crimp in response to changes in humidity. It has long been attempted to impart such function to synthetic fibers, and for example, Patent documents 1 and 2 have proposed forming side-by-side composite fibers using nylon-6 and polyethylene terephthalate. However, these composite fibers have not been actually employed because of the minimal reversible variation in the percentage of crimp in response to changes in humidity.
Patent documents 3 and 4 later proposed improvements in the heat treatment conditions. Also, Patent documents 5-8 proposed applications of this prior art. However, a practical level of use has not been achieved because of the small degree of variation in percentage of crimp which results after steps such as dyeing and finishing.
On the other hand, Patent document 9 attempts to overcome the problem described above by forming a polyester component and a polyamide component into a flat form and bonding them, in a side-by-side fashion, using a highly hygroscopic polyamide such as nylon-4 as the polyamide component but, because of the poor reeling stability of nylon-4 and reduced crimping performance resulting from heat treatment, there has been a limit to the practicality of even this type of composite fiber.
[Patent document 1] Japanese Examined Patent Publication SHO No. 45-28728
[Patent document 2] Japanese Examined Patent Publication SHO No. 46-847
[Patent document 3] Japanese Unexamined Patent Publication SHO No. 58-46118
[Patent document 4] Japanese Unexamined Patent Publication SHO No. 58-46119
[Patent document 5] Japanese Unexamined Patent Publication SHO No. 61-19816
[Patent document 6] Japanese Unexamined Patent Publication No. 2003-82543
[Patent document 7] Japanese Unexamined Patent Publication No. 2003-41444
[Patent document 8] Japanese Unexamined Patent Publication No. 2003-41462
[Patent document 9] Japanese Unexamined Patent Publication HEI No. 3-213518